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Credit: Photos: VSL

Top: Although post-tensioning has been used for nearly 50 years, older systems focused more on obtaining the desired prestress force and less on durability. The industry has evolved to offer systems that deliver the desired prestress force with improved protection for the prestressing steel. Bottom: The elevated deck of the Consolidated Rental parking garage at BWI Airport was constructed using a post-tensioned concrete beam and slab system.
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With unbonded post-tensioning, the prestressing steel is installed on the jobsite just before concrete is poured. The prestressing steel is greased and encased in an extruded plastic sheathing to prevent it from bonding to the concrete.
Applying Bonded Post-Tensioning

While monostrand post-tensioning systems are used in the majority of post-tensioned concrete building applications, bonded systems are becoming more popular with long-term owners, such as airports, hospitals, government agencies, and universities. Approximately 3.5 million square feet of bonded post-tensioned building slabs have been installed in United States since 1995.

One key design feature of bonded systems is that the hardened grout locks the movement of the post-tensioning strands to that of the surrounding concrete. Hence, the force in a bonded strand is a function of the deformation of the surrounding concrete. This is the concept of strain compatibility and internal equilibrium used in reinforced concrete design.

Another design advantage of bonded post-tensioning is the inherent capacity for resistance to progressive collapse. This may be especially important in the event of localized blast loading. Like mild steel reinforcement, a bonded post-tensioning tendon can develop its force at a relatively near distance along its length. If an anchorage fails or a strand is severed, the loss of tendon force is localized. The remainder of the tendon would retain its force at the development length away from the failure point and would remain functional. This functionality may be used in the design phase when planning for alternative load paths.

Bonded systems also offer several practical benefits, such as reduction of mild steel, particularly at the top of slabs. Alleviating steel is especially important because most parking garage maintenance costs are due to repairs associated with spalled concrete and corroded rebar. Another benefit is complete encapsulation, since the strands are fully protected by cementitious grout, duct, and surrounding concrete. The bonded systems also offer flexibility in terms of structural modification for stairwell openings, utility access, and future expansion.

The Buck Stops Here

Beyond longevity and flexibility, today's parking professional also must address initial and long-term costs of materials and construction technologies. As such, Manassas, Va.-based WDP Associates conducted a life-cycle cost analysis between bonded and unbonded systems in 1999 for the construction of elevated parking decks and beams. Costs associated with initial construction and future repair costs were used to compare the life cycles of the different systems for a 50-year expected life span. A wide variety of variables were considered, including estimated repair costs, concrete deterioration rate, and exposure to deicing chemicals. The analysis indicated that substantial life-cycle cost savings are possible when a bonded post-tensioning system is selected in lieu of a traditional, unbonded post-tensioning system.

Choosing VSL's VSLAB bonded post-tensioning system for the BWI garage, planners selected a design concept for the 3.5 million square feet of parking space—more than 1 million square feet of which is elevated, post-tensioned, cast-in-place concrete. A one-elevated-level concept was selected to allow all rental car companies to operate from the same level. The structure is approximately 1500x840 feet, and is founded on spread footings that are up to 6 inches thick, 16 feet wide, and 16 feet long.

The slab-on-ground is 6 1/2 inches thick (reinforced with welded-wire fabric) with contraction joints at 20 feet on center each way. The elevated deck consists of a post-tensioned concrete one-way beam and slab system. Slabs 5 1/2 inches thick span 20 feet between beams that span 60 feet to girders or columns. Concrete compressive strength at 28 days is 5000 psi. Overall stability is provided by moment connections between the columns and beams and girders.

The System

The VSLAB system totally encapsulates strands using high-density plastic duct with watertight mechanical duct to anchorage couplers. Permanent end caps for both beam and slab tendons are included to completely seal the anchorages. High-performance grout pumped through the tendons provides an additional layer of protection.

The post-tensioning system favorably met the garage's functionality requirements. While standard commercial parking garages typically have bays measuring 20 to 30 feet wide and 54 feet long, the column grid spacing for this project was increased to 60x60 feet to allow more flexibility for the rental companies. According to Walker Parking project manager Jason Gross, “The larger bay spacing was important because each rental group wanted as much clear space at the bottom level as possible to allow for staging and multidirectional traffic.” Also, the floor-to-floor height was increased 19 feet to give customers the feel of an open structure.

Strict control of stressing and grouting operations was required due to the large quantity of strand involved (more than 3 million linear feet) and the aggressive placement schedule. Slab tendons were partially stressed (10 kips/strand) on the day after concrete placement to help control shrinkage cracking and completely stressed once concrete test cylinders—cured under jobsite conditions—reached a compression strength of 3200 psi. Partial stressing of beam and girder tendons was not required. These tendons were fully post-tensioned when the concrete compressive strength reached 3200 psi.